The present invention relates to a pneumatic radial tire (hereinafter referred to as a radial tire) for a passenger vehicle, and more particularly, to a radial tire which improves the steering stability on a dry road (hereinafter referred to as dry performance) without deteriorating the wet performance.
Heretofore, with a view to improving the durability of the belt portion and to reducing the rolling resistance, radial tires have been proposed in which the carcass line is defined by an "equilibrium carcass line".
This equilibrium carcass line means a natural equilibrium configuration of a carcass layer formed as a result of the balance between the normal internal pressure of a tire and a reaction force generated in an area where the carcass layer and a belt layer overlap with each other and the tension of the carcass layer in a condition wherein no substantial force other than the normal internal pressure and the reaction force acts on the tire when the tire is inflated up to its normal internal pressure.
For instance, British patent specification No. BP 2,006,695B proposes the improvement of the durability by providing the equilibrium carcass line such that a natural equilibrium configuration formed when no substantial force other than the internal pressure of the tire acts thereon is exhibited in an area extending from the maximum width position of the carcass layer toward the bead portion when the tire is inflated, and that a theoretical equilibrium configuration formed when no substantial force other than the internal pressure and the reaction force of the belt layer acts thereon is exhibited in the remaining area extending from the maximum width position toward the tread. In addition, the specification of U.S. Pat. No. 4,513,802 proposes the reduction of the rolling resistance by increasing the height of the sidewall of the maximum width position of a tire.
However, in each of these proposals, it is intended to control the distribution of tension of carcass cords, and it is not intended to control the distribution of tension of belt cords, which the present invention proposes to control as hereinafter described. Therefore, the belt tension generated when radial tires proposed in the above cited Patent specifications were inflated up to their normal internal pressures becomes greater at the ground-contacting central portion of the tread portion, but gradually decreases toward the shoulder side end portions, and the apparent rigidity decreases. Because of this, it was not possible to improve the steering stability. Furthermore, the width of grooves in the tread portion is reduced by the side force generated while driving to turn, and therefore it was not possible to avoid the deterioration of the wet performance.
In a radial tire for a passenger vehicle, in consideration of the wet performance, it is common that the ratio of groove area to the total ground-contacting area of the tread portion when loaded with a designed normal load is determined to range from 30 to 40%, and in a radial tire requiring a higher wet performance, this ratio exceeds 40%. However, it is inevitable that the dry performance decreases as the groove area ratio increases, and this increase of the groove area ratio results in the reduction of wear characteristics. Furthermore, in some racing tires requiring a higher dry performance, the groove area ratio is set as 30% or less, but the reduction of wet performance is inevitable, and there exist safety problems in driving on ordinary roads.